Hi Redsun,
thank you for your answer.
Following your advice I spent time again to check every of them.
Everything is tagged as compatible with this motherboard, except for the power supply which is a brand new "Be Quiet! Straight Power 10 400W".
Given the flashing leds, I suspect that many things are OK (if there was a real show stopper, the led would be solid amber as per the manual); the absolute absence of VGA output signal is the strangest symptom: I suppose this should be the BMC output, hence independent from whatever hardware is on the MB except for the PSU of course.
I have few clues about the reasons why there would be absolutely no VGA output signal except for a faulty hardware (bad soldering, etc.).
If you have any ideas for that VGA problem, that would be great!
Thanks & regards,
David